There are different causes of cyanosis in preterm infants, which can be classified as physiological or pathological. Generally speaking, physiological cyanosis, which can sometimes appear within 5 minutes after birth, is due to the fact that the ductus arteriosus and foramen ovale have not yet closed after birth and still maintain a right-to-left shunt, that the lungs have not yet fully opened after birth, that pulmonary ventilation is not perfect, and that the peripheral skin is poorly perfused. After 5 minutes, the general circulatory changes are complete, the arteriovenous flow is completely separated, the color of the lips and nail bed turns pink, and the normal oxygen saturation is restored. This is because the pressure in the right atrium rises and exceeds the pressure in the left atrium, creating a right-to-left shunt through the foramen ovale. This temporary cyanosis may disappear immediately after crying, which is also a physiological cyanosis. Pathological cyanosis can be divided into peripheral cyanosis, central cyanosis, and other causes of cyanosis. Peripheral cyanosis can be caused by a cold environment, high hemoglobin content, or local venous obstruction, due to the slow flow of blood through the capillaries of the peripheral circulation and increased tissue oxygen consumption, resulting in increased local reduction of hemoglobin. In general, this cyanosis can be caused by normal oxygen saturation of blood and partial pressure of oxygen in arteries, or in some systemic diseases such as heart failure where the blood flow in body circulation is slower, or in shock when the heart beat volume is reduced, the blood supply in peripheral circulation is reduced, and the blood flow in capillaries is stagnant. In the case of erythrocytosis, blood viscosity increases, and in the case of sclerosis, hypothermia, blood concentration and reduced cardiac output can slow down blood flow and cause cyanosis. Blood concentration and reduced cardiac output during sclerosis hypothermia can also slow blood flow and cause cessation. There is also a condition called central cyanosis, which is usually caused by heart and lung disease, resulting in a decrease in oxygen saturation and partial pressure of oxygen in the arterial blood. Depending on the cause, it can be divided into pulmonary cyanosis and hematogenous cyanosis. Pulmonary cyanosis is caused by cardiac asphyxia, malformation of the airway, posterior nostril obstruction, RDS, pulmonary insufficiency, pneumonia, congenital diaphragmatic hernia, or continuous fetal circulation. Cardiogenic cyanosis is usually a congenital heart disease with right-to-left shunt. More common in the neonatal period are conditions such as tetralogy of Fallot, macrovascular shift or hypoplastic left heart syndrome, ectopic drainage of pulmonary veins, and in some cases, congenital atresia or severe pulmonary artery stenosis. Other causes include methemoglobinemia, erythrocytosis, and neurologically induced central respiratory failure, which can also cause cyanosis.